The present invention relates to a method of configuring pairs each consisting of logical devices to be controlled by two or more storage control apparatus, a system for processing I/O instructions issued by the storage control apparatus to the pair of logical devices and a storage system for implementing the I/O-processing system. In the case of a logical-device pair in a control domain extended over those of a plurality of storage control apparatus, an attempt may be made to allow accesses to be made to individual logical devices of the pair. In such a case, a host apparatus needs to issue a command to make a transition to a state known as a split state. This command causes each of the logical devices composing the pair to transit to a state in which the access history of one of the logical devices is managed separately from the management of the access history of the other logical device. A technology concerning such a remote copy is described for example in Japanese Patent Laid-open No. Hei 10-333838.
In this case, I/O instructions issued by the host apparatus to the individual logical devices are received and processed by the respective storage control apparatus of the logical devices. Since the I/O instructions issued by the host apparatus to the individual logical devices are processed by the respective storage control apparatus of the logical devices, differences in data between the logical devices of the pair are caused. These differences in data are caused by changes resulting from execution of the I/O instruction. The resulting changes in data are stored in difference-information storage means, which are provided in the storage control apparatus respectively. In order to restore the logical devices, which have been controlled independently of each other so far, back to the original synchronous condition of the logical-device pair, the host apparatus issues a Resync command. When a Resync command is issued, data of the auxiliary logical device is made identical with data of the primary logical device by copying the data of the primary logical device to the auxiliary logical device. Typically, the primary logical device is the pair's logical device of an operation system. On the other hand, the auxiliary logical device, which is the pair' other logical device, is typically a volume having a data image of the logical device of the operation system at a point of time.
In this case, however, some data may not be updated by I/O instructions for updating data received during the split state. If some data is not updated, for the data not updated, the amount of data to be copied and the length of the copy operation time can be reduced by merely merging the changes in data, which have been stored in difference-information storage means, and copying only the result of the merging operation. Assume for example that the process is carried out in execution of a Resync command to copy changes in data under control executed by the primary logical device. In this case, a transfer command is issued to the auxiliary logical device to transfer the auxiliary logical device's data changes stored in the auxiliary logical device to the primary logical device to be merged with the data changes of the primary logical device in order to determine data to be copied as changes in data.